Two police cars were stationed at the Newtown high school, The New York Times reported. Police made the rounds of other schools in the district.
The Sandy Hook students went to a former middle school in neighboring Monroe, 7 miles south of their old school, which remains closed. Officials promised additional security there, the report said.
"I think right now it has to be the safest school in America," Monroe Police Lt. Keith White said.
The Chalk Hill School has been renamed Sandy Hook Elementary School. Pictures and other decorations have been moved from the old school.
There were also signs of the attention Sandy Hook has received since Adam Lanza, 20, gunned down 20 children and six staff members before killing himself. The new Sandy Hook is decorated with snowflakes sent from well-wishers around the world.
Along the road to the school were signs of support. "Welcome," read one neon-green sign with a red heart. Another sign filled with hearts and featuring the Elmo "Sesame Street" character said: "Sandy Hook School -- We Luv U!"
Newtown Superintendent Janet Robinson said Sandy Hook staff decided to keep the school's name the same despite its new location.
She also said many people had worked hard to clean, paint and get the school ready for its reopening 20 days after the Dec. 14 mass shooting at the original Sandy Hook school that left 20 students and six staff members dead.
Monroe offered use of the former Chalk Hill School, which was closed almost two years ago.
"Chalk Hill has been transformed from a middle school to a very cheerful, nurturing elementary school," Robinson told a news conference Wednesday as the school staff gathered for the first time as a whole in the new school and parents and students toured the building.
Police would not say what safety and security measures were taken at Sandy Hook in Monroe, but Robinson said the school felt "extremely secure."
Gov. Dannel Malloy and state Education Commissioner Stefan Pryor were also at the school.
"I am utterly inspired by the courage and the determination of the Sandy Hook teachers," Pryor said. "They are overcoming unimaginable challenges ... in arriving back at school and their commitment and grace is clearly on display."
Former Sandy Hook Principal Donna Page came out of retirement to return to the school she led for more than a decade until 2010.
Page replaces Dawn Hochsprung, who was among the first victims of the killing spree by Adam Lanza, 20.
"I want parents and families enduring the loss of their precious children to know their loved ones are foremost in our hearts and minds as we move forward," Page wrote on the school's website.
"Your strength and compassion has been, and will continue to be an inspiration to me and countless others as we work to honor the memory of your precious children and our beloved staff," her message said.